The management of an efficient and safe highway transportation system requires the collection and analysis of various data concerning the flow of vehicles on the streets and roadways which comprise the highway system. Local, state and national transportation planners utilize this collected data as a basis for future construction of new facilities, the installation of traffic control equipment or improvement of the existing highway system. Additionally, the collected and analyzed traffic information can provide valuable assistance to developers in the planning of housing, retail, and industrial construction.
In addition to being used to collect traffic data for monitoring and planning purposes, traffic sensing devices are also utilized in conjunction with traffic signals and other traffic control devices for the real-time control of traffic.
The conventional method for collecting traffic data involves the use of one or more pneumatic tubes placed across the roadway pavement. Vehicles crossing the tube actuate an impulse switch that operates a counting mechanism. For permanent installations, loop detectors embedded in the roadway are commonly used. A typical loop detector consists of a wire loop placed in the pavement to sense the presence of vehicles through magnetic induction. The ends of the loop are connected to an electronic amplifier usually located at a roadside controller.
Although transportation agencies have been using pneumatic tubes and loop detectors for many years, there remain many problems with these devices. For example, pneumatic tubes are susceptible to damage from braking wheels, roadway conditions, age and lack of proper upkeep. In addition recording errors can be introduced by multi-axle vehicles, improper placement of the tubes, movement of the tubes or by a vehicle parked with a wheel in contact with the tube.
Though loop detectors have been found to perform better than pneumatic tubes, they are impractical for temporary purposes, are more expensive to install, and are difficult to repair or replace. Usually several loops or pneumatic tubes are required to obtain information regarding vehicle speeds, the spacing of vehicles or the identification of types of vehicles, such as trucks, busses, cars, etc. Furthermore, neither loop detectors nor pneumatic tubes is suitable for measuring lateral placement of vehicles in travelled lanes. A knowledge of vehicle lateral placement is important when, for example, the safety effects of lane control devices such as barriers, guardrails, signs, pavement markings, drums, and cones are evaluated. In addition, vehicle lateral placement can be analyzed to evaluate drivers perceptions and reactions to road signs, or to determine if a motorist is driving under the influence of alcohol.